Grating scale position measuring systems precisely measure the movement of an object by observing the light diffracted from a grating attached to the object. FIG. 1 illustrates an example of a grating scale position measuring system 100 that measures the position of an object 110. For the measurements, a reflection grating 120 is mounted on object 110 and illuminated with a collimated light beam 130 from a laser or other beam source 140.
Grating 120 diffracts light beam 140 into a zero order maximum centered on a beam X0, a first order maxima centered on beams X1, second order maxima centered on beams X2, and higher order maxima (not shown). A lens 150 in system 100 receives the diffracted light and focuses light from the first order maxima X1 onto an image plane 170. A spatial filter 160 selectively transmits the light in the first order maxima and blocks the rest of the light diffracted from grating 120.
On image plane 170, light from the first order maximum forms a periodic intensity distribution 175 having a spatial period (or wavelength) that depends on the line spacing of gating 120 and the magnification of lens 150. The location or phase of periodic intensity distribution 175 depends on the location of grating 120. Accordingly, as object 110 and grating 120 move perpendicular to incident beam 140, periodic intensity distribution 175 shifts on image plane 170.
Detectors 180 measure light intensity at spatially separated locations along image plane 170. Differences in the measured intensities at the spatially separated points indicate the location or phase of periodic intensity distribution 175. Accordingly, movement or a phase change in periodic intensity distribution 175 indicates movement of object 110. Detectors 180 measure the phase change of periodic intensity distribution 175 and thereby measure the movement of object 110.
For precise measurements, detectors 180 require a sharp image on image plane 170. In particular, using a spherical lens for lens 150 causes spherical aberrations that blur intensity distribution 175 making it difficult for detectors 180 to measure the phase of intensity distribution 175. An aspheric lens can reduce spherical aberrations, but a standard off-the-shelf aspheric lens minimizes spherical aberrations if the object is at infinity. In system 100, light from first order beams X1 diverge from grating 120 so that the approximation of an object at infinity is inaccurate. Accordingly, even with an aspheric lens, aberrations can cause accuracy problems.
System 100 also has a drawback in that most applications of system 100 require a relatively large distance between lens 150 and image plane 170. For example, when object 110 is a stage for a wafer in an integrated circuit fabrication device, the clearance between object 100 and lens 150 needs to be about 19 mm or more, which leads to an object distance of about 19 mm or more. Additionally, with a reasonable size grating (e.g., a 10 xcexcm pitch), detectors 180 require a magnification of 9xc3x97 or more of the grating pitch to allow measurement of the phase of periodic intensity distribution 175. The clearance and magnification requirements result in a total optical path length of about 200 mm between the object and the image. A 200 mm long measuring device is often too large in space critical systems such as typical integrated circuit fabrication equipment.
Folding mirrors can fold the optical length inside a relatively compact package. One exemplary system employs seven folding mirrors to reduce size of the measurement device. However, the folding mirrors require alignment, which increases manufacturing costs. Additionally, the positions of folding mirrors are subject to drift during use of the measurement system, and periodic recalibration of the measurement system can be inconvenient or unacceptable in some applications.
In view of the drawback of existing grating scale position measuring systems, a system is desired that provides a compact device, does not require complicated mirror alignment, is not subject to measurement drift, and provides a light intensity distribution with a magnification and clarity that permits precise phase measurements.
In accordance with an aspect of the invention, a grating scale position measuring system uses a telephoto lens that includes a pair of aspheric lenses positioned for finite conjugates. An additional magnifying system in the telephoto lens can magnify a periodic intensity distribution (i.e., the image) in the image plane to the size required for accurate phase measurements. The magnifying system can use spherical lenses because the aspheric lenses focus light to within a small aperture in the magnifying system, and the rays through the aperture are sufficiently paraxial to avoid introducing significant spherical aberrations.
One specific embodiment of the invention is a telephoto lens that includes a first aspheric lens and a second aspheric lens positioned to form a subsystem that operates at finite conjugates. In one particular configuration, the first aspheric lens is positioned so that an object is at a focal point of the first aspheric lens, and the second aspheric lens is positioned so that an image of the first aspheric lens is an object of the second aspheric lens. The aspheric lenses can be substantially identical and positioned so that the subsystem provides a real image with unit magnification. A magnifying system, that may include one or more negative lens, can magnify the image from the subsystem.
Another embodiment of the invention is a grating scale measurement system that includes a telephoto lens and a detector. The telephoto lens forms an image of a grating, and the detector measures movement of an intensity distribution that the telephoto lens forms in an image plane. The telephoto lens generally includes multiple aspheric lenses having configurations such as in the telephoto lenses described above.